A fingerprint acquisition system acquires an image of a fingerprint that is in contact with an imaging sensor. Such a sensor could be, for example, a CMOS chip comprising, inter alia, a 2-D pixel array, in which each pixel includes a metal plate having a capacitance that changes when the tip of a finger is in close close proximity to the plate. The small ridges and valleys on the finger tip making contact with the surface of the CMOS chip form variations in capacitance across the chip that can be measured using a known current source to remove a fixed amount of charge from each capacitive plate over a predetermined interval of time. The resulting voltage (pixel signals) outputted by the sensor is then used to form an image of the fingerprint.
There are a number of different ways of outputting the values of the pixel signals from the 2-D pixel array. One way of outputting such pixel values is to read out each row of the array in turn starting from the first row using a so-called column-parallel architecture known in the art to access each row of elements 12. In such an architecture, each column requires its own read-out circuit, which allows each element 12 to be accessed at a line rate rather than at a higher pixel rate. In the ideal case, each column read-out circuit would be identical, i.e., matched. However, due to variations in the processes used to manufacture the fingerprint sensor, the column read-out circuits are not perfectly matched.
Disadvantageously, images produced by such a sensor suffer from fixed column noise, which introduces substantial spatial correlations into the image. This problem may confuse subsequent processing designed to extract relevant features for verification purposes.